Radio frequency identification tags (hereinafter referred to as “RFIDs”) are well-known electronic devices which have uses in many areas including freight transportation and retail product tracking. An RFID works by first recording or “burning in” identification or other data into memory in the RFID device. Thereafter, the RFID sends the recorded identification or other information to an RFID reading device. A particular advantage of RFIDs over bar code, optical characters and magnetic storage (such as the magnetic strip on many credit cards) is that the RFID does not require physical contact, or as is the case with optical character and bar code readers, line of sight, between the tag and the reading device to be read. Some currently available active tags can be read at distances up to 300 feet through typical crates and containers used during shipping.
RFIDs come in two varieties: active and passive. An active RFID includes a battery or other power source, and is activated by a signal from a reading device. The activated RFID then broadcasts its identification or other data, which is picked up by the reading device. Some active tags have memory chips on which data can be written and received remotely. An advantage of active RFIDs over passive RFIDs is that the inclusion of a power source allows the active RFID to transmit to a receiver without entering into an electromagnetic field to power the tag circuit. Active RFIDs are also generally able to transmit over a longer distance and be activated individually. The advantage of active RFIDs has led to its use in automatic toll-paying systems, or the like.
Passive RFIDs have no power supply per se, but power is provided to the RFID circuitry by using an electromagnetic power receiver. The RFID reading device sends power to the RFID's electromagnetic power receiver, thus powering up or turning on the RFID's circuits. Next, the passive RFID broadcasts a response signal containing identification or other information, which is then read by the reading device. Because the passive RFID has no battery, it is less expensive and lighter. Passive RFIDs have been in use for some time, notably in security access cards where the user holds the card near the card reader to unlock a door, and in clothing stores as security tags attached to clothing items.
Active tags can be contacted individually or in groups by a computer equipped with RF transmission capability (hereinafter referred to as a “Reader”). A reader can be a handheld, transportable device or it can be mounted in a fixed position.
Fixed readers work with an antenna array arranged around a portal and read all tags that pass through that portal. Fixed readers can be much more selective in that they read only the tags that pass through the portal. Fixed readers can also tell the direction the tag was moving as it passes through the portal and can do so at speeds at 40 or 50 miles per hour.
Proximity sensors, or sensors that can sense the presence or absence of an object without physical contact are available in many forms. All of these devices have the common functionality in that they generate and transmit or direct energy and receive back portions of that energy as it is reflected back from the target and thereby detect the presence or absence of the target or in this case the door. Some types of these sensors are described below. Hall Effect Sensors employ a magnetically biased semiconductor, Hall element to sense moving objects. Light Feedback or fiber optic sensor: the emitter and receiver can be in the same housing or side-by-side. The emitter sends out a beam of pulsed red or infrared light which is reflected directly by the target (at any angle), it is diffused in all directions and some light is reflected back. The receiver sees only a small portion of the original light, switching the sensor when a target is detected within the effective scan range. Ultrasonic Proximity Sensor: This device allows for alternate transmission and reception of sound waves. The transducer emits a number of sonic waves which are reflected by an object, back to the transducer. After emission of the sound waves, the ultrasonic sensor will switch over to receive mode. The time elapsed between the emitting and receiving is proportional to the distance of the object from the sensor. Radio Frequency Sensors use pulsed RF source for reference RF Power value, the Diode receiver is polled for average baseline value every T seconds. Changes in baseline value reflect movement. All of the above items and similar devices will hereinafter be referred to as “Sensors”.
Shipping containers, crates, cartons, and boxes (hereinafter referred to as “Containers”) have long been a target for theft and are a security threat because not only are items stolen out of them, but terrorist devices or contraband can be placed aside them. In the past shipping containers have been ordinarily protected by the use of locks or other physical impediments to entry. Physical seals have also been used so that a quick visual inspection can determine if the seal has been broken. More recently the seals and locking devices have become more sophisticated and some contain RF devices that broadcast RF alerts when the seal is physically broken. Some of the RFID seals contain fiber optic cables or wires which signal a fault when cut or broken. Some contain micro switches, magnetic latches, or contact switches. All of these require some mechanical or electromechanical device to determine the fault before it is broadcast on the RF communication tag.